Multisite: Correct URL to avoid a 404 of the loading spinner

In views/admin/network-admin-footer.php, the loading spinner is loaded from views/admin/_inc/images/... which does not exist. 

I'm not sure the best way to call on this without hardcoding in the slug, unless we had a constant in jetpack.php similar to JETPACK__PLUGIN_DIR. If that's a better route, I can modify the PR to do that.